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## 6.12 THE SZ40 PIN WHEELS

A pin-wheel is a mechanical implementation of a tape; it generates a periodic sequence of 0's and 1's. A pin-wheel contains a number L of pins equally spaced around its circumference. The pin-wheel operates, so that

• When a pin is active (present), the pin-wheel XORs a 1 to its input;
• When a pin is inactive (absent), the pin-wheel XORs a 0 to its input.

The pin-wheel depicted in Figure 6.17 shows four pin positions without pins. In an actual SZ40, all pin positions had pins, but some were made inactive by folding them down.

The SZ40 had 12 pin-wheels (Fig. 6.18):

• 5 χ pin-wheels, χ1, χ2, …, χ5; the length of the χi pin-wheel is Ti.

Figure 6.17 An SZ40 pin-wheel.

Figure 6.18 The SZ40 pin-wheels.

• 5 ψ pin-wheels, ψ1 ψ2, …, ψ5; the length of the ψi pin-wheel is Si.

• 2 motor pin-wheels: a μ pin-wheel of length 37, and a π pin-wheel of length 61.

We assume that each pin-wheel has an initial position marked by 0.

The key stream k(0), k(1),… was generated according to the following rules:

 KS0. The set of active pins on a pin-wheel was part of the key variable ...

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